The compressor section of a typical gas turbine engine comprises a case circumscribing an engine axis and axially alternating arrays of stationary vanes and rotatable blades. Each vane array may be constructed of multiple vane clusters distributed circumferentially about the interior of the case with each cluster being supported by the case. Each vane cluster comprises a radially inner shroud, a radially outer shroud, and two or more airfoils extending between the shrouds. Collectively, the inner and outer shrouds define the inner and outer boundaries of part of an annular flowpath for a working medium fluid.
During engine operation, the vane clusters are subject to nonuniform heating and cooling. The accompanying temperature gradients can cause damage by overstressing the clusters. To help alleviate these thermally induced stresses, one of the two shrouds may be divided into segments by slots that sever the shroud at locations circumferentially intermediate two neighboring airfoils. Since the outer shroud of each cluster connects the cluster to the case, it is conventional to segment the inner shroud rather than the outer shroud. The slots reduce the risk of damage by allowing the shroud segments to expand and contract independently of each other.
One technique for forming the slots is wire electro-discharge machining (EDM). Wire EDM uses an electrically charged electrode in the form a wire wound around a source spool and extending to a take-up spool. The vane cluster shroud is exposed to the wire between the spools. During the EDM operation, the wire travels from the source spool to the takeup spool and simultaneously advances toward the shroud. The difference in electrical charge between the wire electrode and the shroud causes an electrical discharge that removes material from the shroud. As material is removed, the wire advances through the shroud until the slot is completely formed.
One drawback of the shroud slots is that they provide a path by which working medium fluid can leak out of the flowpath during engine operation or by which non-working medium fluid can leak into the flowpath. Leakage can be mitigated, to some extent, by using a small diameter EDM wire to cut a thin slot, i.e. one with a correspondingly narrow kerf. However the use of thin EDM wire leads to increased machining time. Moreover, thin EDM wire is more susceptible to breakage than thick EDM wire during the EDM operation. Thin EDM wire is also more likely than thick EDM wire to be stalled by the presence of minute particulate impurities trapped in the vane cluster. Finally, commercially available EDM equipment capable of using thin wire is more specialized than EDM equipment capable using thicker wire. As a result, a manufacturer may find it economically unattractive to invest in the more specialized, thin wire equipment. Accordingly, it may be desirable to avoid thin slots in favor of relatively wider slots
One way to reduce leakage through a wide slot is to provide a recess in the interior of the slot and install a seal in the recess. U.S. Pat. Nos. 3,728,041, 3,970,318, and 5,167,485 show arrangements of this type. Although such seals may be easily installable between the shrouds of individual vanes, or between the circumferential extremities of adjacent vane clusters, they are not easily installable in the inter-airfoil shroud slots of an otherwise unitary vane cluster. In addition, forming the intra-slot recess increases manufacturing cost and decreases manufacturing throughput. Another possible way to mitigate leakage is to install an external seal, such as the sealing strip 78 shown in U.S. Pat. No. 4,422,827, to bridge across each slot. However such external seals also increase manufacturing cost.
What is needed is a vane cluster with thermally independent shroud segments and which is economical and easy to manufacture.